Method of making fluorine out-diffused optical device

ABSTRACT

A fluorine containing silicate glass substrate is heated to a temperature sufficiently high to permit fluorine to out-diffuse from the surface thereof. A surface region is formed having a gradient fluorine concentration in a direction perpendicular to the substrate surface, the region of lowest fluorine concentration having the highest refractive index. The resultant device is capable of functioning as an optical waveguide.

BACKGROUND OF THE INVENTION

The present invention relates to graded index optical devices and to amethod of making the same. More particularly, this invention relates toa method of fabricating optical devices such as waveguides by heattreated a fluorine containing glass body to produce a change in therefractive index of a surface region of the body due to fluorineout-diffusion. As used herein, the term "optical waveguide" refers to amaterial containing a system of refractive index gradients capable ofguiding waves of optical energy. This term, therefore, includes bothoptical fibers which are usually employed as the transmission medium foroptical communication systems and to planar devices which are usuallyemployed in optical circuits that are required for processing opticalsignals. The light propagating channel can be a cylindrical core in thecase of an optical waveguide fiber, or it can be a planar layer on thesurface of a substrate or sandwiched between two adjacent layers oflower refractive index. The terms "optical energy" and "optical", asused herein, include the infrared, visible and ultraviolet portions ofthe electromagnetic spectrum.

Some operational theories and other pertinent information concerningoptical waveguide fibers can be found in U.S. Pat. No. 3,157,726 issuedto Hicks et al. and in the publication "Cylindrical Dielectric WaveguideMode" by E. Snitzer, Journal of the Optical Society of America, Vol. 51,No. 5, pages 481-498, May 1961. Information concerning planar opticalwaveguides may be found in the publications: "Evanescent Field Couplinginto a Thin-Film Waveguide" by J. E. Midwinter, IEEE Journal of QuantumElectronics, Vol. QE-6, No. 10, October, 1970, pages 583-590; "LightWaves in Thin Films and Integrated Optics" by P. K. Tien, AppliedOptics, Vol. 10, No. 11, November, 1971, pages 2395-2413; and"Dielectric Rectangular Waveguide and Directional Coupler for IntegratedOptics" by E. A. J. Marcatili, The Bell System Technical Journal, Vol.48, No. 7, September 1969, pages 2071-2102.

Although single mode waveguides are advantageous in that they arecapable of propagating optical signals with very low dispersion, due tothe low numerical aperture and/or small core size of such fibers, lasersmust be employed to inject optical signals into these waveguides.Multimode waveguides generally have larger core diameters and largernumerical apertures than single mode waveguides and are therefore oftenthe preferred medium for the transmission of optical signals, since theycan accept light from incoherent, broad spectral width sources such aslight emitting diodes. However, in a multimode waveguide, the variousmodes propagate at slightly different group velocities. Thus, a shortinput pulse that is shared by a plurality of guided modes splits up intoa sequence of pulses that arrive at the output end of the waveguide atdifferent times. This type of pulse dispersion is the dominant cause ofdispersion in multimode waveguides.

A well-known mode equalization technique which results in decreaseddispersion requires and index gradient across the light propagating coreor channel. For example, assuming a cylindrical waveguide, therefractive index is greatest along the axis thereof and decreases as acertain power α of the radius. A discussion of graded index waveguidesappears in the publications "Optical Fibers for Communication" by D.Gloge, Applied Optics, Vol. 13, No. 2, February 1974, pp. 249-254 and"Multimode Theory of Graded-Core Fibers" by D. Gloge and E. A. J.Marcatili, Bell System Technical Journal, Vol. 52, No. 9, November 1973,pp. 1563-1578. As a result of this variation in refractive index acrossthe optical waveguide core or channel, light rays deviating from theaxial direction propagate into regions of lower index where their higherspeed compensates for the greater distance of propagation. Thus, asgraphically illustrated on page 252 of said Gloge publication, theimpulse response of graded index optical waveguides is substantiallyimproved.

Gradient index optical waveguides are also advantageous in that theyhave less light scattering loss because boundary imperfections aresmoothed out. In planar waveguides surface imperfections at the boundarybetween the light transmitting layer and air, for example, stillcontribute to scattering, but the modal optical power is concentratedaway from the surface of the planar structure. Therefore, loss due toscattering at that surface is reduced.

SUMMARY OF THE INVENTION

Briefly, the present invention relates to a method of forming an opticaldevice having a layer, the refractive index of which is higher than thatof the remainder of the device. A transparent silicate glass body isprovided the body containing an amount of fluorine effective to decreasethe refractive index thereof to a value below that of the silicate glassalone. The glass body is heated to a temperature below the softeningpoint temperature of the silicate glass but sufficiently high to effectan out-diffusion of fluorine from at least one surface of the glass bodyto form at that surface a layer of reduced fluorine content. Theconcentration of fluorine in the surface layer increases from a minimumvalue at the surface to a maximum value within the body so that therefractive index of the surface layer decreases from a maximum value atthe surface to a minimum value within the body.

The surface layer is of adequate thickness and has a refractive indexsufficienty greater than that of the remainder of the glass body toenable the layer to propagate optical wave energy. If the surface layeris formed on the inner surface of a glass tube, the tube can be drawninto an optical waveguide fiber.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a planar optical waveguide constructed in accordancewith the method of the present invention.

FIG. 2 is a graph illustrating fluorine concentration as a function ofdistance from the surface of the waveguide of FIG. 1.

FIG. 3 is a graph illustrating the variation in refractive index withrespect to distance from the surface of the waveguide of FIG. 1.

FIG. 4 is a cross-sectional view of a planar optical waveguide.

FIG. 5 is a tubular blank which can be employed in the formation ofoptical waveguide fibers.

FIG. 6 is a graph illustrating the refractive index as a function ofdistance across the tube of FIG. 5.

FIG. 7 illustrates the step of drawing a tubular blank into an opticalwaveguide fiber.

FIG. 8 illustrates the refractive index distribution across an opticalwaveguide fiber formed in accordance with the present invention.

DETAILED DESCRIPTION

It is to be noted that the drawings are illustrative and symbolic of theinvention and there is no intention to indicate scale or relativeproportion of the elements shown therein.

In accordance with the present invention, an optical device having asurface layer of high refractive index relative to that of the remainderof the device is formed by a fluorine out-diffusion process. Referringto FIG. 1, a fluorine containing silicate glass substrate 10 issubjected to a sufficiently high temperature to cause fluorine toout-diffuse from surface 12, thereby forming in that region of thesubstrate adjacent to surface 12 a fluorine depleted layer 14 having arefractive index greater than that of the remainder of the substrate. Afluorine containing substrate having this type of surface layer iscapable of functioning as a planar optical waveguide.

Substrate 10 can be formed from any fluorine containing silicate glasscomposition that lies in a stable glass forming region, i.e., it can beformed from a glass which does not devitrify or opalize when thefluorine is added or during the fluorine out-diffusion heat treatment.If the fluorine causes the glass to devitrify or if it causes theprecipitation of fluorides, light scattering can occur in the glass,thereby rendering it unsuitable for optical application. Thus, fluorinemay be present in amounts effective for forming a high refractive indexsurface layer, but it must be present in amounts sufficiently low thatdevitrification or precipitation of fluoride crystals does not occur.All silicate glasses investigated showed an inverse relationship betweenfluorine concentration and refractive index; however, this relationshipbetween refractive index and fluorine concentration was not observed inborate and phosphate systems. Relatively low levels of fluorine presentin the base silicate glass were sufficient to permit the formation ofoptical waveguides by a fluorine out-diffusion process. Alkali aluminoborosilicate glasses have been found to be very satisfactory forpurposes of the present invention, these glasses being easily melted andexhibiting relatively low thermal expansion. Some base glasses to whichfluorine may be added to form substrates suitable for use in the presentinvention are set forth in Table I wherein compositions are expressed inweight percent on the oxide basis as calculated from the batch. Inaccordance with conventional practice the silver in composition H, whichwas added to the batch as silver nitrate, is expressed in percent byweight in excess of the total glass composition in which the sum of theconstituents listed other than silver totals 100%.

                                      TABLE I                                     __________________________________________________________________________    A         B    C    D    E    F    G    H    I    J                           __________________________________________________________________________    SiO.sub.2                                                                          51.9 56.3 54.8 64.1 64.5 60.6 60.0 63.7 54.8 63.5                        Al.sub.2 O.sub.3                                                                   9.6  9.0  10.6 8.0  --   9.6  10.0 12.1 19.0 12.1                        B.sub.2 O.sub.3                                                                    23.5 16.3 16.2 17.1 21.0 19.8 20.0 16.0 17.0 16.0                        BaO  --   6.7  8.4  --   --   --   --   --   --   --                          PbO  1.0  5.1  5.4  5.1  1.7  1.6  --   --   --   --                          Li.sub.2 O                                                                         2.5  2.6  2.7  1.0  --   3.5  --   2.0  1.5  2.0                         Na.sub.2 O                                                                         9.2  1.8  1.9  3.5  11.5 3.6  10.0 6.2  7.5  6.2                         K.sub.2 O                                                                          0.4  --   --   1.2  1.3  1.3  --   --   --   --                          Sb.sub.2 O.sub.3                                                                   --   --   --   --   --   --   --   --   0.2  0.2                         CdO  1.9  --   --   --   --   --   --   --   --   --                          CuO  *    --   --   --   --   --   --   --   --   --                          ZrO.sub.2                                                                          --   2.2  --   --   --   --   --   --   --   --                          Ag   --   --   --   --   --   --   --   2.5  --   --                          __________________________________________________________________________     *indicates a trace amount                                                

To form a planar optical waveguide, a flat sheet or plate offluorine-containing glass is formed by any one of the various well knowntechniques, and one surface thereof is ground and polished to form aflat, optical quality surface, the opposed surface also being groundflat. The resultant substrate is heat treated in a dust-free environmentto cause fluorine to out-diffuse from the optical quality surface.During heat treatment the substrate is preferably supported on anoptically flat refractory oxide or refractory metal support. To causefluorine out-diffusion to occur, the substrate is generally heat treatedat a temperature between the annealing point and the softening point.The annealing point generally corresponds to a viscosity of 10¹³ poiseand the softening point to a viscosity of about 10⁷.6 poise. Thesoftening and annealing point temperatures generally depend upon glasscomposition. For example, the softening points of compositions A, B, Dand F of Table I are 638°C, 676°C, 666°C and 660°C, respectively, andthe annealing points thereof are 481°C, 506°C, 460°C and 489°C,respectively. Temperatures above the softening point can be employed,but the glass tends to deform under its own weight, and preservation ofan optically flat surface becomes difficult. Some fluorine out-diffusiontakes place at temperatures lower than the annealing point, and it isexpected that some out-diffusion will take place at temperatures as lowas the strain point, wherein the viscosity is equal to about 3 × 10¹⁴poise. However, for out-diffusion to take place within reasonableperiods of time, temperatures above the annealing point should be used.For purposes of comparison, it is noted that the strain pointtemperatures of compositions A, B, D and F of Table I are 447°C, 476°C,420°C and 456°C, respectively.

FIG. 2 illustrates the relative concentration of fluorine as a functionof distance from surface 12 as a result of fluorine out-diffusion due toheating. FIG. 3 illustrates the manner in which the refractive indexvaries with respect to distance into the substrate from surface 12 as aresult of the fluorine concentration gradient illustrated in FIG. 2. Theeffective waveguide thickness, which is illustrated in FIG. 3 by thedashed line, is illustrated by dashed line 16 in FIG. 1. Since therefractive index decreases gradually from a high value at the surface toa relatively lower value at some distance from the surface, multimodewaveguides which exhibit relatively low dispersion due to velocitydifferences among the modes can be produced by this method.

The following example illustrates the manner in which planar opticalwaveguides can be formed in accordance with the method of the presentinvention. To a glass melt having composition A of Table I was added0.47 wt.% fluorine. Fluorine content is expressed as percent by weightin excess of the total weight of the other composition constituents. Themelt was cast into a plurality of flat substrates, the surfaces of whichwere thereafter ground and polished. The effects of the duration of theheat treatment as well as the temperature thereof are illustrated inTable II.

                  TABLE II                                                        ______________________________________                                                     Heat Treatment    No. of                                         Composition  Temp.     Time        Modes                                      ______________________________________                                        Base glass A to                                                                            500°C.                                                                           88 hrs.     none                                       which 0.47 wt.%                                                                            "         136 hrs.    1                                          fluorine was added                                                                         550°C.                                                                           1 hr.       none                                                    "         16 hrs.     1                                                       "         64 hrs.     2                                                       600°C.                                                                           1/2 hr.     1                                                       650°C.                                                                           10 min.     1                                          ______________________________________                                    

After a substrate was heat treated, a beam of laster light having awavelength of 6328 A was coupled into the fluorine depleted surfacelayer by means of a glass prism in the manner taught in U.S. Pat. Nos.3,586,872 and 3,822,928. Table II gives the number of modes that couldbe propagated in the surface layer as a result of different heattreatments. It is known that the number of modes that can be propagatedin the surface layer is indicative of the thickness of that layer andthe difference in refractive indices of the surface layer and the bulkportion of the substrate. Table II indicates that the fluorineout-diffusion process is both temperature and time dependent. When arelatively low heat treatment temperature, viz. 500°C, was employed, aninsufficient amount of fluorine out-diffusion occurred and the resultantdevice did not function as an optical waveguide even after 88 hours ofheat treatment. However, by extending the heat treatment time at 500°Cto 136 hours, a single mode waveguide was formed. By increasing thetemperature to which base glass A was heated to 650°C, a single modewaveguide could be formed in 10 minutes.

To illustrate the effect of the initial concentration of fluorine in thebase glass as well as the effect of the duration of the fluorineout-diffusion process, base glass composition A of Table I was used toform five different melts. Each melt contained an amount of fluorinewhich is set forth in the left column of Table III.

                  TABLE III                                                       ______________________________________                                                  Number of modes propagated as a                                     Fluorine  result of heat treatment at 550°C                            Concentration                                                                           for specified number of hours                                       of each melt                                                                            16 hrs.  36 hrs.   56 hrs. 124 hrs.                                 ______________________________________                                        0.1 wt.%  0 modes  0 modes   1 mode  1 mode                                   0.2 wt.%  0 modes  0 modes   1 mode  1 mode                                   0.4 wt.%  1 mode   1 mode    2 modes 2 modes                                  0.8 wt.%  1 mode   2 modes   2 modes 3 modes                                  1.2 wt.%  1 mode   2 modes   2 modes 3 modes                                  ______________________________________                                    

Each melt was used to form four substrates of the type illustrated inFIG. 1. The substrates were subjected to heat treatment at 550°C fortime periods ranging from 16 hours to 124 hours, as indicated in TableIII, to form in each substrate a surface layer of lower fluorineconcentration than the remainder of the substrate. A beam of laser lighthaving a wavelength of 6328 A was coupled into the fluorine depletedlayer in the manner previously described. Table III indicates that thenumber of modes which could be propagated in these waveguides increasedwith increasing fluorine concentration in the base glass as well as withincreased duration of the high temperature fluorine out-diffusion heattreatment.

The refractive indices of the fluorine depleted surface layers of thesubstrates cannot be measured. However it is well known that therefractive index of a light guiding layer must be higher than that ofthe surrounding media; for example, see the article entitled"Transmission of Optical Energy Along Surfaces: Part II InhomogeneousMedia" by H. Osterberg et al., Journal of the Optical Society ofAmerica, Vol. 54, pp. 1078-1084. To further illustrate the relationshipbetween fluorine content and refractive index, various amounts offluorine were added to base glass compositions H, I and J of Table I,and the refractive indices of the resultant glasses are indicated inTable IV.

                  TABLE IV                                                        ______________________________________                                               Refractive Index                                                       wt.% F   Base Glass H                                                                              Base Glass I                                                                              Base Glass J                                 ______________________________________                                        0        1.494       1.494       1.492                                        1.5      1.492                                                                2.5      1.488                                                                3.0                              1.483                                        5.0      1.478                   1.477                                        10.0     1.474                   1.473                                        15.0                 1.471       1.473                                        ______________________________________                                    

Table IV reveals an inverse relationship between fluorine content andrefractive index. However, the change in refractive index is morepronounced at lower fluorine concentrations, and the refractive indexappears to saturate at the higher levels of added fluorine as shown bycomposition J which had the same value of refractive index when both 10and 15 wt.% fluorine was added to the batch. The maximum amount offluorine that can be incorporated in the glass is not known, and eventhough 15 wt.% fluorine was added to the batch, the entire amount maynot have remained in the glass. If composition J were to be employed,there would be no benefit of adding more than 10 wt.% fluorine to thebatch since no further decrease in refractive index could be obtained,at least with the melting conditions employed. Referring again to TableIII, it is seen that relatively small amounts of fluorine are sufficientto effect a sufficient change in refractive index to form opticalwaveguides. Thus the larger fluorine concentrations shown in Table IVare not needed for that application. The inverse relationship betweenfluorine content and refractive index is also disclosed in U.S. Pat. No.3,784,386 issued to R. J. Araujo et al.

As shown in FIG. 4 a protective layer 18 of transparent material may beapplied to the surface of light guiding layer 14. Layer 18 must have arefractive index lower than that of layer 14 and must be formed at atemperature sufficiently low that the high index layer is not adverselyaffected. Layer 18 could be a sputtered oxide layer, a plastic layer ofthe like.

Optical waveguide fibers can also be formed by a fluorine out-diffusionprocess. A fluorine-containing glass tube 20, which is illustrated inFIG. 5, is subjected to heat treatment to cause out-diffusion offluorine from the surfaces thereof. The temperature and length of timeto which tube 20 is subjected to heat treatment determines the amount offluorine out-diffusion which occurs. This heat treatment causes theinner and outer surfaces of tube 20 to contain fluorine depleted regionsof the type represented in FIG. 2. Thus, the relative refractive indexas measured across a section of tube 20 is represented by the graph inFIG. 6. The regions of highest refractive index are the inner and outersurfaces of tube 20 which contain the lowest concentration of fluorineafter heat treatment.

After the high temperature out-diffusion process is completed, tube 20is heated to the drawing temperature thereof and is drawn as illustratedin FIG. 7. Fibers can be drawn at temperatures such that the viscosityof the glass is in the range of 10⁴ to 10⁷ poise. For composition B ofTable I, for example, this temperature range would be between 700°C and950°C. As tube 20 is drawn, the cross-sectional area thereof is reducedand inner surface 22 collapses, thereby forming an optical waveguidefiber 24 having a solid cross-section. As fiber 24 is drawn, thecross-sectional area thereof can be decreased until the desired fiberdimensions are obtained. In accordance with well known techniques, thefinal reduction in fiber diameter can be achieved in one or more redrawsteps.

Working in the aforementioned drawing viscosity range, significantout-diffusion of fluorine can take place in a matter of minutes. Sincethe uncollapsed tube can be in the hot zone of the drawing furnace for aperiod of time on the order of minutes, the tube can be subjected to asingle heat treatment to simultaneously out-diffuse fluorine from theinner and outer tube surfaces and permit the tube to be drawn into afiber.

Once the inner surface collapses, the glass is elongating at such arapid rate that the temperature remains above the annealing point foronly a matter of seconds. This is an insufficient amount of time tocause the fluorine gradient to level out by further diffusion. a processwhich, if permitted to occur, would destroy the waveguide.

FIG. 8 illustrates the relative refractive index of fiber 24 across asection perpendicular to the axis of the fiber and along a diameterthereof. Points 32 and 34 on refractive index curve 30 represent theeffective boundary between the core and cladding of waveguide fiber 24.Since tube 20 is subjected to a high temperature during the drawingthereof, additional fluorine diffusion will take place after the innersurface 22 collapses, thereby rounding off the central portion of curve30.

Although the present invention has been described with respect tospecific details of certain embodiments thereof, it is not intended thatsuch details be limitations upon the scope of the invention exceptinsofar as set forth in the following claims. It is obvious that thefluorine out-diffusion process, which has been specifically describedherein in connection with the formation of optical waveguides, can alsobe employed to form other optical devices having layers of highrefractive index.

We claim:
 1. A method of forming a planar optical waveguide comprisingthe steps ofproviding a flat substrate of transparent silicate glasscontaining an amount of fluorine effective to decrease the refractiveindex thereof to a value below that of the silicate glass alone,polishing a surface of said substrate to form an optical qualitysurface, and heating said substrate to a temperature below the softeningpoint temperature but greater than the annealing point temperature ofsaid silicate glass, said temperature being sufficiently high to effectan out-diffusion of fluorine from said surface to form at said surface alayer of reduced fluorine content, the concentration of fluorine in saidlayer gradually increasing from a minimum value at said surface to amaximum value within said substrate so that the refractive index of saidlayer decreases from a maximum value at said surface to a minimum valuewithin said substrate.
 2. The method of claim 1 wherein said silicateglass contains at least 0.1 weight percent fluorine.
 3. A method inaccordance with claim 1 further comprising the step of forming on thesurface of said layer of reduced fluorine content a protective layerhaving a refractive index less than that of said layer of reducedfluorine content.
 4. A method of forming a glass optical waveguide fiberhaving a refractive index distribution throughout its length such thatin a plane perpendicular to the fiber optical axis the refractive indexprogressively decreases from a higher value at the optical axis of thefiber to a lower value at some finite fiber radius, said methodcomprisingproviding a hollow tube of transparent silicate glasscontaining an amount of fluorine effective to decrease the refractiveindex thereof to a value below that of the silicate glass alone, heatingsaid tube to a temperature below the softening point temperature butgreater than the annealing point temperature of said silicate glass,said temperature being sufficiently high to effect an out-diffusion offluorine from at least the inner surface thereof to create at said innersurface a layer of reduced fluorine content, the concentration offluorine in said layer gradually increasing from a minimum value at saidinner surface to a maximum value within said tube, said fluorineout-diffusion process causing the refractive index of said tube togradually decrease from a high value at the inner surface of said tubeto a lower value within said tube, subjecting said tube to a temperaturesufficiently high to permit said tube to be drawn into a fiber, anddrawing said tube to collapse the hole in the center thereof and toreduce the cross-sectional area thereof and thereby form a glass opticalfiber having a refractive index distribution throughout its length whichprogressively decreases from a high value at the optical axis of saidfiber to a minimum value at some radius within said fiber.
 5. The methodof claim 4 wherein said silicate glass contains at least 0.1 weightpercent fluorine.
 6. A method in accordance with claim 4 wherein thesteps of heating and subjecting are simultaneously performed.